The Origins of Egg Shape

THE COMMON MURRE, a slender, northern-dwelling bird that resembles a miniature penguin, nests on cliffs and frigid island coasts. Its marble-patterned eggs are highly asymmetrical, almost conical, a structure that, according to a popular theory of egg shape, might have evolved as an adaptation to the bird’s rocky nesting terrain: conical eggs spin in a tight circle, making them less likely to roll off a cliff and shatter. Other hypotheses link egg shape to clutch size, or to diet—birds with limited access to calcium might lay spherical eggs, requiring the least surface area per unit of volume.

A recent analysis[4] of egg shape, published this week in Science, provides support for a less popular theory: that the evolution of adaptations for flight also drove variation in egg shape. Across hundreds of species, the strongest predictor of a bird species’ egg shape was not diet or habitat but how strong a flyer it is, according to the study led by Princeton biologist Mary Caswell Stoddard, JF ’16, and Lakshminarayanan “Maha” Mahadevan, England de Valpine professor of applied mathematics, of organismic and evolutionary biology, and of physics. Compact avian bodies pared down to accommodate long-distance flights, writes Cambridge zoologist Claire Spottiswoode in an accompanying commentary, may “need to negotiate their narrower pelvis, and because the only way to fit a chick into a narrower egg is to make the egg longer, elliptical or asymmetric eggs result.” The murre, with its wide migration ranges, fits the narrative neatly.

“Bird eggs are a wonderful model system” for comparative biologists, says Stoddard, who met Mahadevan while she was a junior fellow. “The egg stage for birds is a very tenuous one—there’s a high risk of something going wrong—so we expect that selection pressures have acted strongly on eggs. That’s why bird eggs are terrific for asking questions about form, function, and biological constraint.” The murre’s eggs are “famous in the egg-laying world” not just for their pointy shape, Stoddard explains, but also their coloration, “one of the most intricate pigmentation patterns in the world. There are ideas about whether murres lay eggs that are individually recognizable to their parents.”

“My colleagues and I found that despite a lot of interest in egg shape and many hypotheses, most of these hypotheses had been generated for specific groups of birds, and we lacked a global, comprehensive analysis of egg shape across all birds,” Stoddard says. In the present study, she and Mahadevan, along with co-authors Ee Hou Yong, Derya Akkaynak, Catherine Sheard, and Joseph Tobias, used egg-shape data from 1,400 species (about 14 percent of all bird species) to classify egg shape along two dimensions: ellipticity (the egg’s deviation from a perfectly spherical form) and asymmetry (the difference in diameter between one end and the other). Murre eggs are high in both ellipticity and asymmetry; owl eggs, which look almost spherical to the naked eye, are low in both. A chicken’s egg lies somewhere in the middle, Stoddard explains, “on the low end of asymmetry and average-to-low in ellipticity.”

The team then modeled how a shape emerges, treating the egg as an expanding, pressurized elastic shell. Variation in egg shapes grew out of differences in pressure applied along the egg’s surface, and differences in the membrane’s thickness. The more uniform the membrane’s thickness, the more spherical its shape. “We were able to manipulate just two basic parameters—variation in the properties of the membrane and variation in pressure across the membrane—to generate the whole diversity of egg shape,” Stoddard says.

Senior author Mahadevan (who was recently appointed[5] a faculty dean of Mather House) links egg shape to his broader interest in how biological form and function influence each other; he has previously used simple math models[6] to understand folds in the brain, a drying paint drop, wrinkles in the skin, or ripples in a leaf. “Why an egg?” he asks. “People have wondered about egg shape for millennia, quite literally…It’s a relatively simple shape to model—it’s got an axis of symmetry and therefore can be described effectively by a planar curve. There are a very large number of shapes, and there’s a large range of biological [specimens] that actually exist.” The researchers relied on the digitized egg collection at the Museum of Vertebrate Zoology at the University of California, Berkeley, Stoddard says, the largest egg collection in North America, with more than 50,000 samples collected during the nineteenth and twentieth centuries.

The final stage of the project, which looked at associations between egg shape and traits associated with different bird species, produced the most surprising result. The hand-wing index (HWI), a measure of wing shape that biologists use as a proxy for flight ability, had by far the strongest correlation with asymmetric and elliptical egg shapes. “The HWI is correlated with migration behavior and dispersal distance,” Stoddard explains. “Asymmetry and ellipticity would allow birds to pack more volume into an egg that has a fixed width”—to fit more egg into a narrow space streamlined for flying. Future studies might consider whether anatomical traits like pelvic width also correlate with HWI and egg shape. Or there might another factor entirely, still unaccounted for, that flight-adapted birds have in common.

Flight ability was the strongest predictor of egg shape on a global scale—a revision to the current literature. But within smaller subsets of the data, other factors became important. Among shorebirds, chick precociality (a technical term, Stoddard assures), or how mature a chick is immediately after hatching, was one of the best predictors of egg asymmetry. “We think that eggs that are really pointy may also have a very broad blunt end, which may have an increased density of pores, and these increased pore densities may allow for more oxygen to reach the developing brain of an advanced precocial chick—perhaps that’s what’s happening with shorebirds,” she suggests. “Our study detracts nothing from other hypotheses—we acknowledge that there are other selective forces that are operating on smaller taxonomic scales.”

Relating egg shape to anatomical measurements, like pelvic width and oviduct size, will be a topic for immediate follow-up. The findings also may contribute to more ambitious theory, about the broader arc of evolution. “Birds are special in that they lay asymmetric eggs. The only other non-avian vertebrates that lays asymmetric eggs appear to be some theropod dinosaurs,” Stoddard says, now very excited. “So this dinosaur-to-bird transition is particularly interesting to us, because it looks like this is the point at which egg asymmetry may have evolved. And if that coincides with the emergence of powered flight, there may be some connection there.” The study of eggs reminds her that human beings, and mammals in general, all descend from egg-laying ancestors: “The amniotic egg was critical to the evolution of land vertebrates—it was a specialized egg that’s not unlike the egg laid by modern birds, that allowed our vertebrate ancestors to move out of the water and colonize land.”